How Many Galaxies Exist? When astronomers chose a relatively empty spot on the sky near the Big Dipper and used the Hubble Space Telescope to record and extremely long exposure of 10 days duration, they found thousands of galaxies crowded into the image. Known as the Hubble Deep Field. Other deep fields have been imaged since. The GOODS (Great Observatories Origins Deep Survey) has used the Hubble Telescope with other space telescopes such as the Spitzer Space Telescope and the Chandra X-ray Observatory to image selected deep fields.
Types of Galaxies 3 main types of galaxies: Spiral Disk-shaped usually consisting of spiral arms, containing gas and dust. Elliptical No disk, no spiral arms, and almost no gas and dust. Irregular Generally shapeless and tend to be rich in gas and dust.
Galaxy Classification 0 Elliptical: Spiral: Sa Sa = Large nucleus; tightly wound arms E0 = Spherical E1 Sb Sc Sc = Small nucleus; loosely wound arms E7 = Highly elliptical E6
Galaxy Classification Roughly 2/3 of all spiral galaxies are barred-spiral galaxies, classified SBa, SBb, and SBc. Show a pronounced bar structure in the center. Our own Milky Way galaxy is an example of this.
Neighboring Irregular Galaxies Large Magellanic Cloud Some smaller galaxies (“dwarfs”) orbit larger galaxies. Example: The Large Magellanic Cloud and Small Magellanic Cloud are 2 of 11 known dwarf galaxies orbiting our Milky Way galaxy. Small Magellanic Cloud
Distance to Galaxies The distances to galaxies are so large that it is not convenient to measure them in light-years, parsecs, or even kiloparsecs. Instead, astronomers use the unit Megaparsec (Mpc), or 1 million pc. To find the distance to a galaxy, astronomers must search among its stars, nebulae, and stars clusters for familiar objects whose luminosity or diameter they know. Such objects are called distance indicators. Most are objects whose brightness is known, an astronomers refer to them as standard candles.
The Hubble Law In the 1920’s, Edwin Hubble and Milton Humason measured the distances to a number of galaxies using Cepheid variable stars, stars changing their brightness with a period of 1-60 days where the period of variation is directly related to its luminosity. Published a graph in 1929 that plotted the apparent velocity of recession versus distance for their galaxies. The straight-line relation from Hubble’s diagram can be written as a simple equation. This relation between red-shift and distance is known as the Hubble Law. Hubble Law Formula Vr = Hd Vr = apparent velocity of recession, in km/s H = Hubble constant ~ 70 km/s/Mpc d = distance, in Mpc
The Hubble Law
The Hubble Law Example 1: A galaxy is calculated to be 5 Mpc away. What is its apparent recessional velocity? Example 2: A galaxy has an apparent recessional velocity of 700 km/s. What is the distance to this galaxy? 350 km/s 10 Mpc
Distance Pyramid Hubble’s Law Standard Candle/ Tully-Fisher Variable Stars (Cepheids) Spectroscopic Parallax Stellar Parallax Radar Ranging